Parthenium hysterophorus L., commonly known as parthenium poses a major health problem and a threat to Agriculture in South Africa. This is based on studies in other parts of the world where it has been shown to inhibit the growth and seed germination of other plants through allelopathy (the chemical inhibition of one organism by another), and also to cause asthma and serious dermatitis in humans. The sesquiterpene lactone parthenin is a major allergen of the species.

Although it was first recorded in Natal, South Africa, in 1880, it appears to have become common and troublesome only since the 1980’s. Its current distribution, based on herbarium specimens in the Pretoria National Herbarium as well as recent sightings is shown in the accompanying map.It extends from the subtropical coastal belt of KwaZulu-Natal, northwards to Swaziland, Mozambique, and the southern section of the world-famous Kruger National Park, to the savanna north west and north of Pretoria to Zimbabwe.

Parthenium is an aggressive colonizer of wasteland, roadsides, railsides, watercourses, cultivated fields and overgrazed pastures. In the subtropical lowlands of South Africa it has become a major nuisance in sugar cane and banana plantations. It spreads from seeds which are adapted to dispersal mainly by water and to a lesser extent by wind. Most long-distance dispersal is by animals, vehicles, farm machinery and in mud.

Parthenium has recently been declared a category one weed in South Africa which means that it is a prohibited plant and that steps must be taken to control it or eradicate it where possible. A herbicide has been registered in South Africa for the chemical control of this species.